Lithium-ion secondary batteries have a high energy density and a large capacity, and are therefore widely used as driving power supplies for mobile data terminals, such as mobile phones and notebook computers. Recently, lithium-ion secondary batteries have been expected to be used in applications where high voltage and large capacity are required, such as driving power supplies for battery-driven automobiles and home use power storage systems.
A lithium-ion secondary battery includes a flammable organic solvent, and therefore the safety of the battery needs to be ensured. Accordingly, a sealing body that seals the battery is provided with a gas venting mechanism that vents gas in the battery to the outside of the battery when the battery internal pressure increases.
A technology regarding a gas venting mechanism according to the related art will be described with reference to FIG. 10. FIG. 10 is a sectional view of a sealing body having a structure of the related art.
A sealing body of a sealed battery according to the related art includes a valve cap 21 having vent holes 21a, a PTC thermistor 22, a pair of explosion prevention valves 23 and 25 having rupture grooves 23a and 25a that rupture when the battery internal pressure increases, an insulating plate 24 that prevents peripheral portions of the pair of explosion prevention valves 23 and 25 from coming in to electrical contact with each other, and a terminal plate 26 that has vent holes 26a and that is electrically connected to a positive plate. With this technology, when the battery internal pressure increases, first, the electrical contact between the pair of explosion prevention valves 23 and 25 is disconnected, so that the current path to the valve cap 21 is interrupted. When the battery internal pressure further increases, the rupture grooves 23a and 25a formed in the explosion prevention valves 23 and 25 rupture, so that holes are formed. And then, the gas in the battery is vented to the outside of the battery through the vent holes 26a, the holes formed in the pair of explosion prevention valves 23 and 25, and the vent holes 21a. 
PTLs 1 to 3 listed below describe technologies for increasing the safety of the battery.